Electrodes for obtaining uniform discharges in electrically pumped gas lasers

ABSTRACT

Uniform &#39;&#39;&#39;&#39;glow-type&#39;&#39;&#39;&#39; electrical discharges are established in gaseous laser media at atmospheric pressure through use of dielectric electrodes consisting of ferroelectric ceramic materials. These electrodes may be used to produce both volume ionization and molecular excitation of the media or alternatively a second set of electrodes operating transversely to the dielectric electrodes may be used for molecular excitation, i.e., electrical pumping, of the gas volume.

United States Patent [1 1 Erickson et a].

[451 July 10, 1973 1 ELECTRODES FOR OBTAINING UNIFORM DISCHARGES 1N ELECTRICALLY roman GAS LASERS [75] inventors: George F. Erickson; Thomas F.

Stratton, both of Los Alamos, N. Mex.

[73] Assignee: The United States of America as represented by the United States Atomic Energy Commission, Washington, D.C.

MacNair, Study of Electron Emitters for Use in Gas La- CHARGING RESISTOR sets. IEEE J. Quant Elect., Vol. QE-S, No. 9 (September 1969) pp. 460-470.

Primary Examiner--William L. Sikes Attorney-Roland A. Anderson [5 7] ABSTRACT Uniform glow-type electrical discharges are established in gaseous laser media at atmospheric pressure through use of dielectric electrodes consisting of ferroelectric ceramic materials. These electrodes may be used to produce both volume ionization and molecular excitation of the media or alternatively a second set of electrodes operating transversely to the dielectric electrodes may be used for molecular excitation, i.e., electrical pumping, of the gas volume.

'4 Claims, 9 Drawing Figures samura Fig.

CHARGING RESISTOR STORAGE Z /6 CAPACITOR Fig.2

CHARGING RESISTOR 4 HVW SPARK A Fig.3

CHARGING SPARK RESISTOR GAP RESON ATOR ELECTRODES FOR OBTAINING UNIFORM DISCHARGES IN. ELECTRICALLY PUMPED GAS LASERS BACKGROUND OF THE INVENTION directed toward mechanical subdivision of the discharge electrodes, with separate current sources for eachsubdivision. Resistive current limiters, fed from a common energy source, represent the usual form of this arrangement, although separate energy sources (condensers) are also successful. Recently, the problem has been attacked differently through separation of the mechanism which produces the current carriers (ionization) from the mechanism of molecular excitation (inelastic electron collisions). Volume ionization is produced by irradiation of the gas with a flux of high energy electrons, while the simultaneous application of an electric field below the breakdown limit of the gaseous lasing media pumps energy into the molecules.

The use of dielectric materials in conjunction with electrodes in electrically pumpedgas lasers is well known in the art. The literature discloses numerous socalled double discharge" lasers in which at least one electrode is covered with a dielectric material. See, e.g., A. K. Laflamme, Double Discharge Excitation for Atmospheric Pressure CO Lasers," 41 Rev. Sci. Instr. 1578 (1970). The presence of the dielectric favors uniformity of electron emission and prevents the formation of arcs. Unfortunately, the low energy densities which canbe accommodated without damage to the dielectric materials heretofore disclosed has limited the use of such dielectric covered electrodes to the formation of the preliminary discharge used to preionize the laser gas, after which the gas is electrically pumped by a much more energetic discharge between other electrodes.

SUMMARY OF THE INVENTION We have now found that uniform, high-energy, glowtype discharges can be produced between dielectric electrodes consisting of ferroelectric ceramic materials. Particularly useful for this purpose are ferroelectric ceramics consisting substantially of barium titanate. Ferroelectric ceramic electrodes may be used to produce both volume ionization and molecular excitation of a high-pressure lasing media or alternatively a second set of electrodes operating transversely to the dielectric electrodes may be used for molecular excitation, i.e., electrical pumping, of the gas volume.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows the construction of a commercial high voltage filter capacitor from which the dielectric electrodes'used to obtain the experimental data herein disclosed were derived.

FIG. 2 shows afirst circuit which may be used with the ferroelectric electrodes of thisinvention.

FIG. .3 shows a second circuit which may be used with the ferroelectric electrodes of this invention.

FIG. 4 shows a third circuit which may be used with the ferroelectric electrodes of this invention.

FIG. 5 shows rail-type ferroelectric electrodes useful forureducing corner sparks and streamers.

FIG. 6 indicates the volume resistivity of a heliumcarbon dioxide mixture as a function of CO concentration.

FIG. 7 is aschematicof an equivalent circuit for excitation of gases throughdielectric layers.

FIG. 8 is a schematic representation of the electrodegas geometry in oneembodiment of this invention.

FIG. 9 indicates the energy density in ferroelectric materialsasa function of working stress.

DESCRIPTION OF THE PREFERRED EMBODIMENT Dielectric constant Dielectric strength 18 kV/cm Curie temperature 33C Energy Density (18 kV/cm) mJ/cc Discharges were initiated between electrodes 4 facing ferroelectric ceramic discs 5 1.9 cm in diameter and 2 cmthick using the electrical circuits shown in FIGS.

2 through4. The separation between the faces of the ceramic discs 5 (gas discharge region) was 1 or 2 cm. These circuits respectively represent pulse charging from a storage capacitor(FIG. 2), pulse discharge after dc charging (FIG. 3), and breakdown and charge from an LC ringing circuit (FIG. 4).

The currents shown in FIGS. 2 through 4 all produce discharges reasonablyfree from spark channels. Those streamers and spark elements which are present are principally associated withthe rim of the ceramic at the ceramic-resin interface. The stress accumulation and discontinuities in dielectric constant responsible for the spark channels and streamers can be substantially reducedby fabricating strip or rail ceramic electrodes of the general type shown in FIG. 5. Each rail 8 is composed of a ceramic ferroelectric and is attached to a metal electrode 9. Agaseous lasing media 10 is placed in the volume between rails 8.

The resistive impedance of a gas load between ferroelectric ceramic electrodes spaced 2 cm apart and havchannels with negligible resistive impedances for these gas compositions at this pressure.

Ferroelectric ceramic electrodes may be used not only to ionize the lasing gas mixture but also to pump energy into it. The excitation of a gaseous load contained between two dielectric layers may be analyzed by reference to FIG. 7. As the initial conditions, all capacitors are uncharged. The high voltage is applied to C through R so that the voltage on C increases toward V with a time constant R C At a voltage V, the gas S breaks down, connecting the load to the storage capacitor C The gas breaks down, current passes through C R, and C until the voltage across the load V, is equal to the voltage on C,,. If /2 C C this voltage is nearly V. An analysis of such charging, or dis-- charging circuits, shows that An energy is dissipated in the gas load (neglecting the impedance of the switch 8,) which equals the energy stored in the dielectric layers. In principle, if one is able to open S and close S the energy A C V, stored in the dielectric layers is discharged through R, and S allowing the gas to be excited with 100 percent effi ciency from the power supply, neglecting switch losses and dissipation in the dielectric. In practice, it is difficult to open S, in a time short enough to be of interest.

FIG. 8 is a schematic representation of the electrodegas geometry. As a consequence of the charging pulse, there is deposited in lasing gas 7 an energy per unit area which is 2 (KE /8 )d= W 8 where KE /8 W, is the energy density in the ferroelectric and W, is the energy-density in the gas. Hence we see that for a single charging pulse, the energy density in the gas 7 is The energy densities possible with ferroelectrics are shown in FIG. 9. It appears probable that improvements over the rated performance of the Sprague Type 715C ferroelectric capacitors can be readily achieved. The Sprague capacitors are tested against failure at 1.5 times the rated working stress. In addition, the dielectric constant can be increased to perhaps 6000 by hot pressing the ceramic rather than the present practice of cold pressing with suitable binder material (Point in FIG. 9).

The breakdown stress of laser gas mixtures at atmospheric pressure is approximately the same but somewhat less than the working stress of bulk ferroelectric ceramics. This means that d/o in Eq. (3) can be of the order of unity, and that gas energy densities in the range of 200 to 300 mJ/cc are possible with a single charging or discharging cycle of the ferroelectric electrodes.

What we claim is:

1. In an electrically pumped gas laser, the improvement consisting of first and second electrodes of a bulk ferroelectric ceramic, said electrodes having substantially parallel flat surfaces of said ceramic exposed to the laser gas, and means operatively connected to said electrodes forproducing a potential across said electrodes sufficient to provide substantial ionization within the volume of laser gas to be electrically pumped.

2. The laser of claim 1 having means operatively connected to said electrodes for producing a potential across said electrodes sufficient to provide both volume ionization and molecular excitation of the laser gas.

3. The laser of claim 1 wherein said bulk ferroelectric ceramic is substantially barium titanate.

4. The laser of claim 1 wherein said lasing gas is a mixture of He, CO and N 

2. The laser of claim 1 having means operatively connected to said electrodes for producing a potential across said electrodes sufficient to provide both volume ionization and molecular excitation of the laser gas.
 3. The laser of claim 1 wherein said bulk ferroelectric ceramic is substantially barium titanate.
 4. The laser of claim 1 wherein said lasing gas is a mixture of He, CO2, and N2. 